I have an old MacBook which cannot run anything newer than 10.7.5 Lion. I do regular Time Machine backups to an external drive. I plan to buy a new MacBook Air, but I want to make the migration as smooth as possible, with all my apps, configs and files copied and set just like on the old Mac. I was thinking of re-installing Mountain Lion on the new Air and selecting the old Lion backup as "from backup" source during installation. Will it work? Has anyone tried this and can share experience/problems?

Or maybe I should downgrade Air to Lion (how?), re-install from backup, then upgrade to ML?

2 Answers
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When a newer Mac migrates data from an older OS, the system has upgrade scripts to make sure that the data lands properly on the new system.

These scripts handle all Apple software and perhaps a few high profile programs in case that Apple has found a need to clean up other data.

Migration the other way isn't tested, no scripts or support is provided and in fact, the system normally detects you are moving data backwards and refuses to restore in the first place.

You should be fine restoring a Lion backup onto any Mac with Mountain Lion. You don't need to reinstall anything - just don't run the Setup Assistant until you have the external drive and can move everything to the new mac.

Running an older OS on a newer Mac is also problematic, since often drivers are missing and again, Lion will check and refuse to install on hardware that is too new in general.

The best way is to use the Setup Assistant the first time you start your new Mac. This will copy the data either from the backup or directly from the old Mac. This will copy all of your users and the file ownerships will be set up correctly. If you do go Mac-to-Mac be sure to use an ethernet cable because it will take days using wireless. I migrated a Lion MBP to a new Mountain Lion MBP last summer and it worked very well for me.